Crystal Structure & Mineralogy Flashcards

Which crystal system is characterized by three axes of equal length meeting at right angles?

Answer: Cubic (Isometric)

The cubic (isometric) system has three equal axes (a1, a2, a3) all at 90° to each other, producing highly symmetrical forms like the cube and octahedron.

On the Mohs hardness scale, which gemstone has a hardness of 9?

Answer: Corundum (Ruby/Sapphire)

Corundum, which includes ruby and sapphire, has a Mohs hardness of 9, second only to diamond (10) on the scale.

What term describes the tendency of a mineral to break along flat planes related to its crystal structure?

Answer: Cleavage

Cleavage is the tendency of a mineral to break along smooth, flat planes that reflect the internal crystal structure and atomic bonding.

Diamond belongs to which crystal system?

Answer: Cubic (Isometric)

Diamond crystallizes in the cubic (isometric) system, forming octahedral, cubic, and dodecahedral crystals due to the symmetric arrangement of carbon atoms.

Which property describes the ratio of a gemstone's weight to the weight of an equal volume of water?

Answer: Specific Gravity

Specific gravity (SG) is the ratio of a mineral's density to that of water at 4°C, and it is a key diagnostic property for gemstone identification.

Emerald is a variety of which mineral species?

Answer: Beryl

Emerald is the green gem-quality variety of the mineral beryl (Be3Al2Si6O18), with its color caused by trace amounts of chromium and/or vanadium.

What is the name for a crystal defect where a foreign atom replaces an atom in the crystal lattice, often causing color?

Answer: Isomorphous Substitution

Isomorphous substitution occurs when a foreign atom of similar size and charge replaces a host atom in the crystal lattice, a primary cause of color in many gemstones.